1. Field of the Invention
The present invention relates to an elevator system for transferring a boat during the manufacture of semiconductor devices. More particularly, the present invention relates to an elevator system for transferring a wafer boat such that the wafers in the a boat are maintained horizontal.
2. Description of the Related Art
In general, the fabrication of a semiconductor device involves the execution of a plurality of processes. Therefore, various kinds of equipment and auxiliary devices are used to fabricate a semiconductor device.
These processes include a diffusion process in which a semiconductor device is provided with certain electrical characteristics. The process of diffusion includes a step of forming an oxide film on a wafer, and an annealing step in which the surface of the wafer is exposed to a solid or gas containing boron, phosphorus or the like under a high temperature.
A prior art furnace (FIG. 1) in which the diffusion process takes place comprises a quartz tube 19 and a heating chamber 18 for heating the quartz tube. The quartz tube 19 isolates the wafers 10 from the environment while the diffusion process is being carried out.
The auxiliary devices include an elevator 17 for loading/unloading a boat 14 of wafers 10 into/from the furnace, a wafer transfer device 13 for transferring the wafers 10 into and out of the boat 14, a cassette transfer device 12 for transferring a cassette 11 containing a plurality of the wafers 10 to the wafer transfer device 13, a control unit (not shown) for controlling the above-mentioned devices, and a gas supply unit for injecting refined gas into the heating chamber 18.
In addition to the boat 14, the conventional elevator system comprises a base 15 disposed under and supporting the boat 14, an elevator 17 for moving up and down, and a horizontal control plate 16 interposed between an upper plate of the elevator 17 and the base 15. The horizontal control plate 16 is manually adjustable to control the horizontal attitude of the base 15.
More specifically, as shown in FIG. 2, four hex head bolts 20 extend through the horizontal control plate 16 and into the base 15. The bolts 20 are arrayed along a pair of X and Y axes. The attitude of the wafers 10 with respect to the horizontal is controlled by rotating the hex head bolts 20 clockwise or counterclockwise and thereby adjusting the angle between the base 15 and the horizontal control plate 16. A respective pair of wing nuts 21 are provided at opposite sides of each hex head bolt 20 to fasten the bolt 20 and prevent the bolt 20 from becoming loose.
The boat 14 should extend vertically from and at a right angle to the base 15. However, the boat can become inclined when overloaded or due to pressure from the exterior environment. If such a situation were not attended to, the wafers would not be processed properly, and the equipment could be damaged. Accordingly, the operator periodically loosens the wing nuts 21 and rotates the hex head bolts 20 to manually adjust the base 15 by eye. Once the base 15 is adjusted and the boat is perceived by the operator to extend perfectly vertically, the operator tightens the wing nuts 21 to lock the bolts 20 in place.
However, the task of adjusting the base 15 is onerous because the operator must rotate the hex head bolts one or more times depending on his visual acuity, and the work of adjusting the hex head bolts and of loosening and tightening the eight wing nuts is very time-consuming.